Evolution of the Geysers (US) - Data from Fluid-Inclusion Microthermometry and Gas Geochemistry

Abstract

The Geysers, California, is the site of an active hydrothermal system that initially developed between about 1.5 and 2 Ma in response to intrusion of a hypabyssal granitic pluton. Mineralogic and fluid-inclusion data demonstrate that the present vapor-dominated regime evolved from an earlier and more extensive, liquid-dominated hydrothermal system. Circulation of these early fluids produced veins characterized by tourmaline and/or biotite actinolite clinopyroxene within the pluton and adjacent biotite-rich hornfels, actinolite ferroaxinite epidote, and epidote chlorite wairakite within the intermediate parts of the thermal system, and calcite in the outer parts. Potassium feldspar and quartz are present in all assemblages. Maximum pressure-corrected homogenization temperatures and apparent salinities of fluid-inclusions in these veins range from 440?Cand 44weight percent NaCl equivalent within the hornfels m from the pluton) to 325?Cand 5 weight percent NaCl equivalent at approximately 1500m from the intrusion. We suggest that the shallow, moderate-salinity fluidsare crustal waters modified by water-rock interactions and that the high-salinity fluids are magmatic brines. The formation of vapor-dominated conditions is reflected in the abrupt appearance of low salinity (0.0 to 0.4 weight percent NaCl equivalent) fluid inclusions with homogenization temperatures near 265?C. These inclusion fluids are thought to represent steam condensate formed as the liquid-dominated system boiled off.